For space travel, whether between the Earth and the Moon or traveling among the planets (or any of its moons), it's often a good idea to minimize the propellant mass needed by your spacecraft and its launch vehicle. In a typical spaceflight scenario, a spacecraft will fire its rocket engines to accelerate in order to reach some planned velocity, and will then, when those rocket engines are shut off, continue under its own inertia with the same speed and trajectory it has attained.
It is useful when traveling between planets in the solar system to consider that spacecraft as being in orbit around the Sun. The trajectory with minimal propellant usage is a transfer orbit in which the desired orbit's perihelion (closest approach to the Sun) will be at the distance of Earth's orbit and its aphelion (farthest distance from the sun) will be at the distance of Mars' orbit or of some other destination planet. (Likewise, for journeys to the inner planets of Mercury or Venus, the aphelion will coincide with Earth's orbit and the perihelion will coincide with the destination planet's orbit. It uses its rocket to accelerate opposite the direction of Earth's revolution around the sun, thereby decreasing its orbital energy.) Thus, in the typical scenario, most of the journey between the planets will then consist of coasting towards its destination with the engines turned off. Then to be captured into the destination planet's orbit, the spacecraft must then decelerate relative to that planet using a retrograde rocket burn or some other means.
The typical accelerate-coast-decelerate scenario works well with unmanned probes. However, this will subject human astronauts to long periods of weightlessness during the coasting phase lasting 6 months or longer, depending on peak velocity. Additionally, long trip times will potentially expose astronauts to both solar and cosmic ray radiation. It is known from previous space missions, especially the experience developed from astronauts spending periods of time on space stations (Skylab, Salyut, Mir, and most recently the International Space Station), that weightlessness has adverse health effects upon humans.
The human body on Earth is constantly sensing the effects of gravity and working against it, largely unconsciously. Our muscles (e.g., calves, quadriceps, buttocks, and the erector spinae surrounding the spinal column) are sculpted by the force of gravity in a state of constant exercise, being perpetually loaded and unloaded as we go about our daily lives, even when we are not really “exercising”. But in the absence of gravity these muscles begin to waste away, and subjects can lose as much as a third of total muscle mass in a little over a week, even when physical exercise is deliberately imposed as part of a strength maintenance regimen. The human heart is the body's most important muscle, but also be comes deconditioned when it no longer has to work against gravity to pump blood vertically to the brain. Likewise, bones dynamically maintain their structure to contend with gravitational forces on Earth, but in weightlessness are subject to space-flight-induced osteoporosis as bone calcium finds its way into the bloodstream and is excreted (also causing renal stones, constipation and psychological depression). Many of the physiological changes undergone during prolonged weightlessness are hypothesized to be at least partially permanent, so longitudinal studies of former astronauts are being conducted, even though the necessarily small sample size makes preliminary results inconclusive at the present time. Spaceflight designers have envisioned a number of ways to create an artificial gravity in space, basically constant acceleration of some form, the seemingly most straightforward of which is from the acceleration due to spacecraft thrust.
In order to slow sufficiently to obtain orbital capture by the destination planet, the spacecraft must use about the same amount of fuel that it used to speed up originally. In general, if we want to reduce the travel time between Earth and Mars or some other planet, the more fuel we will need in order to accelerate the spacecraft to a higher coasting velocity and consequently the more fuel will need upon arrival to slow down in order to enter the planet's orbit and then to land. Maximum fuel usage occurs if one accelerates the spacecraft for fully one-half of the journey, with no coasting phase, and then decelerates over the remaining half of the journey. Present spacecraft systems use liquid fuels that constitute a very large percentage of the overall mass. If one could save fuel in some way, while still achieving the desired acceleration, coasting velocity and deceleration, one could shorten travel time or carry more passengers and/or cargo.
Several projects have explored the possibility of nuclear spacecraft propulsion. The first of these was Project Orion from 1958-1963 built upon general proposals in the 1940s by Stanislaw Ulam and others, in which external atomic detonations would form the basis for a nuclear pulse drive. Later, between 1973 and 1978, Project Daedalus of the British Interplanetary Society considered a design using inertial confinement fusion triggered by electron beams directed against fuel pellets in a reaction chamber. From 1987 to 1988, Project Longshot by NASA in collaboration with the US Naval Academy developed a fusion engine concept also using inertial confinement fuel pellets but this time ignited using a number of lasers. Naturally, these last two projects depend upon successfully achieving nuclear fusion.
Muon-catalyzed fusion was observed by chance in late 1956 by Luis Alvarez and colleagues during evaluation of liquid-hydrogen bubble chamber images as part of accelerator-based particle decay studies. These were rare proton-deuteron fusion events that only occurred because of the natural presence of a tiny amount of deuterium (one part per 6000) in the liquid hydrogen. It was quickly recognized that fusion many orders of magnitude larger would occur with either pure deuterium or a deuterium-tritium mixture. However, John D. Jackson (Lawrence Berkeley Laboratory and Prof. Emeritus of Physics, Univ. of California, Berkeley) correctly noted that for useful power production there would need to be an energetically cheap way of producing muons. The energy expense of generating muons artificially in particle accelerators combined with their short lifetimes has limited its viability as an Earth-based fusion source, since it falls short of break-even potential.
Another controlled fusion technique is particle-target fusion which comes from accelerating a particle to sufficient energy so as to overcome the Coulomb barrier and interact with target nuclei. To date, proposals in this area depend upon using some kind of particle accelerator. Although some fusion events can be observed with as little as 10 KeV acceleration, fusion cross-sections are sufficiently low that accelerator-based particle-target fusion are inefficient and fall short of break-even potential.
It is known that cosmic rays are abundant in interplanetary space. Cosmic rays are mainly high-energy protons (with some high-energy helium nuclei as well) with kinetic energies in excess of 300 MeV. Most cosmic rays have GeV energy levels, although some extremely energetic ones can exceed 1018 eV. FIG. 5 shows cosmic ray flux distribution at the Earth's surface. In near-Earth space, the alpha magnetic spectrometer (AMS-02) instrument aboard the International Space Station since 2011 has recorded an average of 45 million fast cosmic ray particles daily (approx. 500 per second). The overall flux of galactic cosmic ray protons (above earth's atmosphere) can range from a minimum of 1200 m−2 s−1 sr−1 to as much as twice that amount. (The flux of galactic cosmic rays entering our solar system, while generally steady, has been observed to vary by a factor of about 2 over an 11-year cycle according to the magnetic strength of the heliosphere.) Outside of Earth's protective magnetic field (e.g. in interplanetary space), the cosmic ray flux is expected to be several orders of magnitude greater. As measured by the Martian Radiation Experiment (MARIE) aboard the Mars Odyssey spacecraft, average in-orbit cosmic ray doses were about 400-500 mSv per year, which is an order of magnitude higher than on Earth.
Cosmic rays are known to generate abundant muons from the decay of cosmic rays passing through Earth's atmosphere. Cosmic rays lose energy upon collisions with atmospheric dust, and to a lesser extent atoms or molecules, generating elementary particles, including pions and then muons, usually within a penetration distance of a few cm. Typically, hundreds of muons are generated per cosmic ray particle from successive collisions. Near sea level on Earth, the flux of muons generated by the cosmic rays' interaction by the atmosphere averages about 70 m−2 s−1 sr−1. The muon flux is even higher in the upper atmosphere. These relatively low flux levels on Earth reflect the fact that both Earth's atmosphere and geomagnetic field substantially shields our planet from cosmic ray radiation. Mars is a different story, having very little atmosphere (only 0.6% of Earth's pressure) and no magnetic field, so that muon generation at Mars'surface is expected to be very much higher than on Earth's surface. Planetary moons, such as Phobos and Deimos around Mars, would experience similar high levels of cosmic ray flux.
In recent years, there have been proposals to send further spacecraft to Mars in 2018 and then manned space vehicles to Mars by 2025. One such development project is the Mars Colonial Transporter by the private U.S. company SpaceX with plans for a first launch in 2022 followed by flights with passengers in 2024. The United States has committed NASA to a long-term goal of human spaceflight and exploration beyond low-Earth orbit, including crewed missions toward eventually achieving the extension of human presence throughout the solar system and potential human habitation on another celestial body (e.g., the Moon, Mars).
It is generally expected to take about nine months to travel to Mars. To get to Mars in less time would require that one burn the rocket engines longer to achieve a higher coasting velocity, but this uses more fuel and isn't feasible with current rocket technology. Likewise, to provide a constant acceleration from thrust (one of the possible artificial gravity schemes) would require the rocket engines burn constantly over the entire flight, leading to even more fuel usage. Even using the standard accelerate-coast-decelerate trajectory, the spacecraft has an overall payload of 100 metric tons, calling for a significant weight penalty in fuel for its liquid rocket engines. Once Mars orbit is reached, the vehicle is too massive to rely upon parachutes and/or a “sky crane” tethered system to descend to the Martian surface. Supersonic retro-propulsion using thrust from large rocket engines are expected to do the job.
The advancing of propulsion technologies would improve the efficiency of trips to Mars and could shorten travel time to Mars, reduce consumables and mass of materials required for the journey, and reduce astronaut health risks from both weightlessness and radiation exposure. Sustained investments in early stage innovation and fundamental research in propulsion technologies is required to meet these goals.
This research and development activity is expected to proceed in several general stages, beginning with an Earth-reliant stage with research and testing on the ISS of concepts and systems that could enable deep space, long-duration crewed missions, followed by a proving ground stage in cis-lunar space to test and validate complex operations and components before moving on to largely Earth-independent stages. Such a proving ground stage would field one or more in-space propulsion systems capable of reaching Mars to undergo a series of shakedown tests to demonstrate their capabilities, select a final architecture, and make needed upgrades revealed by the shakedown tests. While systems already in development for the initial Earth-reliant missions largely make use of existing technologies, investment in the development of newer technologies will be needed to meet the longer-term deep space challenges.